linux-kernel - Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

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Date:   Tue, 31 Oct 2023 14:30:20 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <kvm@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.linux.dev>, <linux-mips@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <kvm-riscv@...ts.infradead.org>,
        <linux-riscv@...ts.infradead.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Xu Yilun <yilun.xu@...el.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Anish Moorthy <amoorthy@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        "Liam Merwick" <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for
 guest-specific backing memory

>+int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
>+{
>+	loff_t size = args->size;
>+	u64 flags = args->flags;
>+	u64 valid_flags = 0;
>+
>+	if (flags & ~valid_flags)
>+		return -EINVAL;
>+
>+	if (size < 0 || !PAGE_ALIGNED(size))
>+		return -EINVAL;

is size == 0 a valid case?

>+int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
>+		  unsigned int fd, loff_t offset)
>+{
>+	loff_t size = slot->npages << PAGE_SHIFT;
>+	unsigned long start, end;
>+	struct kvm_gmem *gmem;
>+	struct inode *inode;
>+	struct file *file;
>+
>+	BUILD_BUG_ON(sizeof(gfn_t) != sizeof(slot->gmem.pgoff));
>+
>+	file = fget(fd);
>+	if (!file)
>+		return -EBADF;
>+
>+	if (file->f_op != &kvm_gmem_fops)
>+		goto err;
>+
>+	gmem = file->private_data;
>+	if (gmem->kvm != kvm)
>+		goto err;
>+
>+	inode = file_inode(file);
>+
>+	if (offset < 0 || !PAGE_ALIGNED(offset))
>+		return -EINVAL;

		goto err;

or hoist the check above fget().

>+
>+	if (offset + size > i_size_read(inode))
>+		goto err;
>+
>+	filemap_invalidate_lock(inode->i_mapping);
>+
>+	start = offset >> PAGE_SHIFT;
>+	end = start + slot->npages;
>+
>+	if (!xa_empty(&gmem->bindings) &&
>+	    xa_find(&gmem->bindings, &start, end - 1, XA_PRESENT)) {
>+		filemap_invalidate_unlock(inode->i_mapping);
>+		goto err;
>+	}
>+
>+	/*
>+	 * No synchronize_rcu() needed, any in-flight readers are guaranteed to
>+	 * be see either a NULL file or this new file, no need for them to go
>+	 * away.
>+	 */
>+	rcu_assign_pointer(slot->gmem.file, file);
>+	slot->gmem.pgoff = start;
>+
>+	xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
>+	filemap_invalidate_unlock(inode->i_mapping);
>+
>+	/*
>+	 * Drop the reference to the file, even on success.  The file pins KVM,
>+	 * not the other way 'round.  Active bindings are invalidated if the
>+	 * file is closed before memslots are destroyed.
>+	 */
>+	fput(file);
>+	return 0;
>+
>+err:
>+	fput(file);
>+	return -EINVAL;

The cleanup, i.e., filemap_invalidate_unlock() and fput(), is common. So, I think it
may be slightly better to consolidate the common part e.g.,

	int ret = -EINVAL;

	...

	xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
	ret = 0;
unlock:
	filemap_invalidate_unlock(inode->i_mapping);
err:
	/*
	 * Drop the reference to the file, even on success.  The file pins KVM,
	 * not the other way 'round.  Active bindings are invalidated if the
	 * file is closed before memslots are destroyed.
	 */
	fput(file);
	return ret;